A 24-year-old woman comes to the emergency department due to 2 days of fever. The patient has no chronic medical conditions, drinks alcohol occasionally, and does not use tobacco or illicit drugs. She is sexually active and frequently travels for her work as a freelance journalist. Temperature is 38.3 C (101 F), blood pressure is 118/66 mm Hg, pulse is 106/min, and respirations are 18/min. Physical examination is notable for mild scleral icterus and splenomegaly. Laboratory findings are shown in the exhibit.
Which of the following infectious agents is most likely responsible for this patient's current condition?

Question

A 24-year-old woman comes to the emergency department due to 2 days of fever.  The patient has no chronic medical conditions, drinks alcohol occasionally, and does not use tobacco or illicit drugs.  She is sexually active and frequently travels for her work as a freelance journalist.  Temperature is 38.3 C (101 F), blood pressure is 118/66 mm Hg, pulse is 106/min, and respirations are 18/min.  Physical examination is notable for mild scleral icterus and splenomegaly.  Laboratory findings are shown in the exhibit.  
Which of the following infectious agents is most likely responsible for this patient's current condition?

Quizplus · Accepted Answer

11ec160b_3842_409d_9249_ffc118c91c1d_MD0001_00 This patient with fever, icterus, splenomegaly, and ring-shaped parasites within red blood cells has malaria, which is caused by 4 main species of Plasmodium (P falciparum, P vivax, P ovale, P knowlesi). Although malaria has been eradicated from the United States, cases still arise due to travel to endemic regions, particularly when no chemoprophylaxis is taken. The parasite is transmitted by female Anopheles mosquitos during blood feeding, travels to the liver where it infects hepatocytes, and then subsequently undergoes asexual reproduction, which generates hepatic schizonts with thousands of daughter cells (merozoites). Rupture of the schizont into the circulatory system leads to erythrocyte infection and cycles of intraerythrocytic reproduction. As the parasite disseminates throughout the circulation, production of inflammatory cytokines causes nonspecific symptoms (eg, fatigue, fever, malaise, headache) and, sometimes, life-threatening complications (eg, seizure, renal failure, circulatory collapse). Hepatosplenomegaly and signs of anemia and hyperbilirubinemia (eg, scleral icterus) are often present. The diagnosis is confirmed when peripheral blood microscopy shows intraerythrocytic trophozoites with a "diamond ring" appearance. (Choice A) Borrelia burgdorferi, the causative organism of Lyme disease, is a spirochete transmitted by the Ixodes tick. It cannot be visualized by conventional light microscopy; diagnosis is made based on clinical symptoms and/or serology. (Choice B) Leishmania tropica, a protozoa transmitted by sandflies, causes a papule at the site of inoculation that evolves over weeks or years into an ulcer with an indurated border. Microscopy can show the oval-round organism within white blood cells. (Choice D) Schistosoma mansoni is a parasitic blood fluke transmitted when humans swim in fresh water contaminated with infected snails. Schistosomes mature in the liver, reside in the mesenteric venules, and release eggs that can cause chronic eosinophilic granulomatous inflammation/fibrosis in organs (eg, bladder, liver, spleen, gastrointestinal tract). Diagnosis is made by identifying eggs in urine or stool, or on biopsy. (Choice E) Toxocara canis, also known as dog roundworm, incidentally infects humans and causes eosinophilic granulomatous inflammation in organs (visceral larva migrans) or the eye (ocular larva migrans). Diagnosis is made by serology. (Choice F) Trichinella spiralis is a roundworm infection that can be transmitted to humans when they ingest larvae (eg, from contaminated pork). Larvae spread through the blood to the muscle and form cysts, causing muscle pain, weakness, and swelling. Diagnosis is usually made by serology or muscle biopsy; the parasite is not intraerythrocytic. (Choice G) Wuchereria bancrofti is a roundworm that is transmitted by mosquitos and causes lymphatic infection (eg, lymphedema, adenolymphangitis). Blood smear (performed in the evening) can often demonstrate the long, thin microfilaria in the circulation; it is not ring-shaped or intraerythrocytic. Educational objective: Malaria is diagnosed when intraerythrocytic diamond ring-shaped parasites are seen on peripheral blood smear. This disease should be suspected in travelers to endemic regions who develop a febrile illness.

A 24-Year-Old Woman Comes to the Emergency Department Due to 2